The hot gas created in the smelting and roasting processes of sulfidic ores and concentrates contains mainly sulfur dioxide, heavy metals, arsenic, halogens and other compounds. It is important that the concentrate bums as completely as possible, and that the oxygen content of the exhaust gas created in the smelting or roasting process is as low as possible, because a high oxygen content in the exhaust gas leads to an increased formation of sulfur trioxide, which leads further to the creation of spent acid in connection with the gas scrubbing. It has been found out that dust, particularly copper-bearing dust, catalyses the formation of SO3. Most probably SO3 is created in the waste heat boiler, and in order to prevent this, it is essential that the oxygen content of the exhaust gas is low and that the amount of leakage air is as small as possible.
The scrubbing of gas created in the smelter or roaster can be divided into dry separation and wet separation. Dry separation methods are realized in connection with a smelter or roaster, in which case the gas exhausted from the furnace gas space is generally first conducted into a waste heat boiler, where a part of the gas heat content is recovered. Thereafter the gas is conducted to an electrostatic precipitator. A significant portion of the heavy metal compounds contained in the gas, excluding mercury and its compounds, separate already in connection with the cooling. The created gas is often conducted to the production of sulfuric acid, and in that case the wet separation of gas takes place in the sulfuric acid plant, in scrubbers, scrubbing towers and wet electrostatic precipitators.
The purpose of the wet scrubbing of gas is to further cool down the said gas adiabatically to a suitable temperature and simultaneously to separate from the gas both solid and volatile impurities, such as heavy metals, halogens, arsenic and selenium. In connection with the wet scrubbing, the SO3 in the gas is washed from the gas as spent acid when it comes into contact with water. The quantity of created spent acid is of the order 1-4% of the amount of sulfur dioxide fed into the wet scrubbing, and its H2SO4 content is of the order 25-35 w-%, otherwise known as dilute spent acid. Wet separation generally takes place as countercurrent scrubbing so that clean water is conducted to the last scrubbing towers, from which it circulates through the scrubbing towers between, to the first tower, to which hot gas is conducted. The cooled, clean gas flow is conducted from the last tower through the wet electrostatic precipitators and drying tower to the production of end products such as sulfuric acid, liquid sulfur dioxide or elemental sulfur. The spent acid obtained from the first tower is conducted to concentration.
The spent acid formed is considered as hazardous waste due to its impurities content, and the costs incurred by waste generation should be as low as possible. The dilute spent acid should be concentrated for final treatment. Concentration is carried out with the aid of evaporators, such as vacuum evaporators and submerged evaporators, to a final concentration of about 70-80 w-% H2SO4. Generally, a vacuum evaporator is used as the first-stage evaporator, where the spent acid is concentrated to a content of about 50 w-% H2SO4. It is clear that, by using a vacuum evaporator, it is possible to concentrate to an even higher content, unless problems are caused by other factors, such as e.g. arsenic crystallization. In most cases concentration is continued by using submerged evaporators, whereupon the above-mentioned final concentration is obtained. Final treatment can be e.g. a practical application or thermal decomposition in a separate furnace. Patent publication Fl-103517 describes a final treatment where concentrated spent acid is fed into a suspension smelting furnace.